The present invention relates generally to a memory control systems, and more particularly to a memory control system capable of interleaving digital signals and of restoring the interleaved signals to the original signals, with a memory of minimal capacity.
In known recording and reproducing apparatuses which record and reproduce an analog audio signal, as it is, on and from a traveling magnetic tape by means of a stationary head, there have unavoidably been a number of problems such as wow and flutter of tape travel and noise and distortion arising in the tape and head system. As a natural consequence, these problems impose limitations when it is desired to increase the quality of recording and reproducing audio signals.
Accordingly, a method which comprises converting an analog audio signal into a modulated digital signal (hereinafter referred to simply as "digital signal") by a modulation method such as pulse code modulation (PCM) and recording and reproducing this digital signal is being developed. By this method, problems such as those relating to the S/N ratio distortion due to nonlinearity of the recording medium are solved to a considerable degree.
For recording and reproducing this digital signal, recording and reproducing systems of wider frequency band or greater number of channels in comparison with those of a systems for recording and reproducing analog signals are required. Accordingly, for this recording and reproducing apparatus, the use of a so-called video tape recorder (hereinafter referred to as VTR) which has heretofore been widely sold on the market as an apparatus for recording and/or reproducing composite video signals has reached a practical stage. The term "composite video signal" is used in the present specification to designate a signal which results from the addition of synchronizing signals such as a vertical synchronizing signal, equalizing pulses, and a horizontal synchronizing signal to a video information signal. A VTR of this type is capable of recording/reproducing video signals of wide frequency band by means of rotary heads which trace with a relatively high relative tracing speed over oblique tracks on a traveling magnetic tape.
Recording of an audio signal by using this VTR is accomplished by converting the analog audio signal to be recorded into a digital signal, and in an adapter apparatus connected to the VTR inserting this digital signal between synchronizing signals which are the same as the synchronizing signals of an ordinary composite video signal, supplying the composite digital signal thus obtained to the VTR, and recording this signal by means of rotary heads on the magnetic tape. At the time of reproduction, the signal reproduced from the magnetic tape by the rotary heads in the VTR is supplied to the adapter apparatus, where the synchronizing signals are removed, and the resulting digital signal is converted into an analog signal, thereby being restored to the original audio signal.
In the case where dust particles adhere to the surface of the above mentioned magnetic tape, or in the case where there are irregularities in the magnetic material of the tape, a signal loss or dropout occurs in one portion of the reproduced signal. In the case where this occurs, and the reproduction of the most significant bit (MSB), for example, is lost, the signal obtained by decoding will have a considerably large erroneous value. If this signal is converted, as it is, into an analog signal and reproduced, it will be accompanied by a high noise voltage, and the reproduced sound will have an unpleasant quality.
As a countermeasure, there has heretofore been adopted an interleaving system. In a digital signal processing system adopting this interleaving system, the input analog signal is sampled at appropriate periods of time in a sampling and holding circuit, and the resulting sampled signal is converted to a modulated digital signal in an analog to digital (A/D) converter. This modulated digital signal is fed to a memory in which data write-in and read-out operation are controlled by a control pulse, whereby the signal is arranged into a combination of words with the words of the series separated from each other by ten-odd H (H denoting the unit horizontal synchronizing period) to be interleaved with each other. Here, a "unit word" refers to the combination of bits obtained from a single sample. The period of time represented by ten-odd H is determined in conformance with the format.
A composite synchronizing signal is added to the signal thus obtained to produce a composite digital signal, which is then recorded on a magnetic tape by a VTR. This rearrangement of the order of data is referred to as "interleaving". Since the interleaving of data results in distribution of the signal information, even when the recording and reproducing signal is deficient over one or more horizontal scanning (H) periods due to dropout, the associated signal present in the separate horizontal scanning period serves to reproduce the normal information.
A data part is extracted from the signal reproduced from the VTR. This data part, which is waveshaped, is then converted into a binary coded digital signal and thereafter supplied to a memory, where it is writen in and read out responsive to a control pulse and is restored to the original order. The resulting signal is supplied to a digital to analog (D/A) converter where it is restored to the original analog signal.
As a memory in the above described recording and reproducing system, there is used a random access memory (RAM) making it possible to conduct the data write-in and read-out operation under control of an external pulse. In order to simplify organization of the memory control system, the total memory capacity of the RAM, which is generally 2.sup.N words (where N is a positive integer), has been divided into 2.sup.X equal segments, and each equally divided segment having a capacity length of 2.sup.N-X (all segments are equal) has been allocated for words which are designated by number ho through hy (where, y.ltoreq.2.sup.X) and should be time divided and multiplexed within 1H. However, when restoring the interleaving or interleaved data to the original normal order (referred to as "deinterleaving" hereinafter), the capacity values (lengths) of the memory capacity required for the respective data allocated by ho through hy differ from each other, which means that the known memory control system requires rather a large number of memory capacity segments and, as a consequence, is not economical. If the case where y=7 is considered, 2.sup.X becomes 8. The data ho is substantially used for deinterleaving, but the part of the capacity length for deinterleaving decreases as the data number increases h1, h2, h3, . . . In the case of h7, no part of the capacity length (corresponding to one eighth of the total memory capacity of memory) is used for deinterleaving. Moreover, as the hatched parts shown for the other data numbers is not required for deinterleaving, there is the disadvantage that about one-half of the total memory capacity is not used effectively.
Moreover, when deinterleaving and jitter elimination processing are carried out simultaneously, the ratio of the non-used part to the total memory capacity of the memory decreases. Furthermore, the non-used memory capacity part increases with an increase in interleaving length.